The a2-pawn is under attack, but even more important is the fact that the queen invasion on d3 cannot be stopped.
18. f3d4  18... c4b2 An excellent move, attacking the pawn on c3 and also preparing Qc4 to activate the queen or exchange it. 19. e2e3 Defending c3. 19... a8c8  20. a1c1 A natural reply, but there's a problem.